I just uploaded another client-side update to display some more information about the issue. When the game starts, type "/debug" into the chat box (without quotes) and it will show you some values on the upper left of the game board.

fps: Frames per second / average frame draw time in msups: State updates per second / average state update time in mssta: Server tick advance

Yeah, that's sad. But as ugly as this looks, I think this is the way to solve the problem. For now, I've disabled the custom window decorations around the client if OpenGL is used. I also uploaded it, give it a try if you want. I'm testing it in my VM

I think I finally got the OpenGL hardware acceleration pipeline working now. It's not yet perfect (in game the sidebar seems to flicker), but on my VM it's already playable again. I really hope that it's working out for you also, at least somehow to be playable, after 24h of frustration ^^